壳层模型分子动力学在CaO研究中的应用

利用壳层分子动力学方法结合有效的对势,研究了高压条件下CaO的熔化曲线。研究表明,分子动力学模拟结果精确地再现了广泛压强范围内CaO的状态方程。研究中考虑了分子动力学模拟熔化存在的过热现象,通过晶体的现代熔化理论,对CaO的分子动力学模拟熔化温度进行了修正,获得了高温高压下CaO正确的熔化温度。因此,常压下引入壳层模型的分子动力学为研究物质熔化提供了一个很好的方法,这种方法可进一步推广到其它物质的高压熔化研究中。     

CaF2熔化温度的分子动力学模拟

利用壳层模型分子动力学方法,研究了高温高压条件下CaF2的熔化温度,同时计算了温度为300K、压强上升到100GPa时CaF2 的状态方程.研究中考虑了分子动力学模拟的过热熔化,通过晶体的现代熔化理论,对CaF2 的分子动力学模拟熔化温度进行了修正, 获得了高温高压下CaF2的熔化温度.因此,常压下壳层模型分子动力学方法为研究物质熔化提供了一个很好的方法.     

自由能方法与零压下Al的熔化温度

利用自由能方法的分子动力学模拟，计算了零压下Al的熔化温度.在计算液相自由能的过程中，采用勒纳-琼斯(LJ)液体作为参考系统，同时将计算结果与Mei和Davenport等人的计算结果进行了比较，计算结果表明：1)选用LJ参考系统使液相自由能的计算时间节省一半，并且不影响熔化温度的计算结果；2)采用不同的埋入原子势(EAM)的分子动力学模拟计算得到的熔化温度与实验值都存在偏差，而就金属Al而言，采用Cai等人的EAM势的熔化温度的计算结果比Mei和Davenport及Morris等人采用的势模型的结果略有改 关键词： 熔化温度 自由能方法 分子动力学模拟     

高压下MgO熔化特性的分子动力学模拟

利用分子动力学方法,模拟研究了高压下MgO的熔化特性．通过晶体的现代熔化理论,对MgO的分子动力学模拟熔化温度进行了修正,得到了高温高压下MgO的熔化温度．计算得到的MgO熔化曲线和已有的实验及其它理论结果在0-135 GPa进行了比较,发现修正得到的MgO熔化温度和由Lindemann熔化方程及两相方法得到的结果在压力低于15 GPa时符合很好．同时,MgO熔化模拟有效解释了一阶相变分子动力学过程中出现的过热熔化现象．     

自由能方法与零压下Al的熔化温度

利用自由能方法的分子动力学模拟，计算了零压下Al的熔化温度.在计算液相自由能的过程中，采用勒纳-琼斯(LJ)液体作为参考系统，同时将计算结果与Mei和Davenport等人的计算结果进行了比较，计算结果表明：1)选用LJ参考系统使液相自由能的计算时间节省一半，并且不影响熔化温度的计算结果；2)采用不同的埋入原子势(EAM)的分子动力学模拟计算得到的熔化温度与实验值都存在偏差，而就金属Al而言，采用Cai等人的EAM势的熔化温度的计算结果比Mei和Davenport及Morris等人采用的势模型的结果略有改     

Molecular Dynamics Simulations for Melting Temperatures of SrF2 and BaF2

利用壳层模型分子动力学方法，考虑萤石结构分子中的预熔化现象，对SrF₂和BaF₂的分子动力学模拟熔化温度进行修正，获得了高压下SrF₂和BaF₂的熔化温度. 同时给出了300 K、0.1 Mpa～7GPa和0.1 Mpa～3 GPa时SrF₂和BaF₂的状态方程，与已有研究结果的最大误差分别为0.3%和2.2%. 计算所得SrF₂和BaF₂常压下的熔点与已有的实验结果符合较好. 对于SrF₂和BaF₂分子体积变化和已有的熔化模拟的差别也做了比较和讨论.     

金属Al表面熔化各向异性的分子动力学模拟

采用嵌入原子势，使用分子动力学方法对金属Al不同低指数晶面的表面熔化现象分别进行了模拟.分析了熔化过程中样品结构组态的变化.模拟结果表明对于不同的自由表面，表面熔化呈现出明显的各向异性行为.Al（110）面在低于熔点的温度之下发生预熔化；（111）与（001）面都出现过热现象.与（111）面不同，（001）面发生过热现象时表面原子层为类液层，而（111）面仍然保持很好的晶格结构，即预熔化的Al（001）面在高于熔点的温度下，仍可以在很长的时间内处于相对稳定的亚稳态.由模拟得到Al的热力学熔点为950 K左右，与实验值基本符合. 关键词： 分子动力学 表面熔化 过热     

三角波加载下金属铝动态破坏现象的微观模拟

采用嵌入原子势模型和分子动力学方法, 模拟研究了三角波加载下金属铝动态破坏的微观过程和动力学性质. 根据原子中心对称参数变化给出了样品微结构演化过程, 解读了熔化前后破坏过程的形态差异; 基于Virial定理统计了样品中压力和温度等力学量波形, 分析了熔化前后材料的强度变化. 通过不同碰撞速度的模拟, 讨论了破碎区内物质形态和密度分布的变化, 给出了材料破坏深度的变化规律. 研究还发现, 熔化后材料的动态拉伸强度已显著降低, 而此时由声学近似推算的材料拉伸强度已明显高于内部应力直接计算结果. 关键词： 破坏 分子动力学 冲击     

金属Cu熔化结晶过程的分子动力学模拟

采用常温、常压分子动力学模拟技术,研究了在周期性边界条件下,由８６４个Ｃｕ原子构成的模型系统的熔化、结晶过程。原子间相互作用势采用ＥＡＭ势。模拟结果表明：在连续升温过程中,金属Ｃｕ在１５２０Ｋ熔化;以不同的冷速进行冷却,在较慢冷却条件下,液态Ｃｕ在１０１０Ｋ结晶;当冷速较快时,液态Ｃｕ形成非晶态。分析了升降温过程中熔体偶分布函数、原子体积、能量、ＭＳＤ随温度的变化特征。     

应变效应对金属Cu表面熔化影响的分子动力学模拟

采用Mishin镶嵌原子势，通过分子动力学方法模拟了金属Cu 的(110)表面在不同应变下的熔 化行为，分析了表面熔化过程中系统结构组态和能量的变化以及固液界面迁移情况.金属Cu 的(110)表面在低于热力学熔点的温度下发生预熔化，准液体层的厚度随温度升高而增加.当 温度高于热力学熔点时，固液界面的移动速度与温度成正比，外推得到热力学熔点为1380K ，与实验结果1358K吻合良好.应变效应（包括拉伸和压缩）导致热力学熔点降低，并促进表 面预熔化进程.在相同温度条件下，准液体层的厚度随应变绝对值的增加而增大.应变效应导 致的固相自由能增加是金属Cu(110)表面热稳定性下降的主要因素，且表面应力和应变方向 的异同也会影响表面预熔化的进程. 关键词： 表面预熔化 热力学熔点 表面应力 分子动力学     

Molecular dynamics of MgSiO3 perovskite melting

The melting curve of MgSiO分子动力学 MgSiO3钙钛矿 熔化温度 高压melting temperature, molecular dynamics, high pressureProject supported by the National Natural Science Foundation of China (Grant Nos 10274055 and 10376021),the Natural Science Foundation of Gansu Province, China (Grant No 3ZS051-A25-027) and the Scientific Research Foundation of Education Bureau of Gansu Province, China (Grant No 0410-01).2005-01-125/8/2005 12:00:00 AMThe melting curve of MgSiO3 perovskite is simulated using molecular dynamics simulations method at high pressure. It is shown that the simulated equation of state of MgSiO3 perovskite is very successful in reproducing accurately the experimental data. The pressure dependence of the simulated melting temperature of MgSiO3 perovskite reproduces the stability of the orthorhombic perovskite phase up to high pressure of 130GPa at ambient temperature, consistent with the theoretical data of the other calculations. It is shown that its transformation to the cubic phase and melting at high pressure and high temperature are in agreement with recent experiments.     

High-pressure melting of MgSiO3

The melting curve of MgSiO(3) perovskite has been determined by means of ab initio molecular dynamics complemented by effective pair potentials, and a new phenomenological model of melting. Using first principles ground state calculations, we find that the MgSiO(3) perovskite phase transforms into post perovskite at pressures above 100 GPa, in agreement with recent theoretical and experimental studies. We find that the melting curve of MgSiO(3), being very steep at pressures below 60 GPa, rapidly flattens on increasing pressure. The experimental controversy on the melting of the MgSiO(3) perovskite at high pressures is resolved, confirming the data by Zerr and Boehler.     

下地幔条件下钙钛矿结构MgSiO3熔化行为的研究

利用分子动力学方法结合有效的对势,模拟了下地幔条件下钙钛矿结构MgSiO3的熔化曲线．研究表明,分子动力学模拟结果精确地再现了广泛压强范围内钙钛矿结构MgSiO3的状态方程,并且熔化曲线与最新的实验结果也符合的很好．在压强上升到下地幔压强范围内,压强低于60 GPa时的钙钛矿结构MgSiO3熔化曲线比较陡,接着变得平缓．在核幔边界压强135 GPa时,钙钛矿结构MgSiO3的熔化温度是6500 K,明显低于Zerr和Boehler实验结果的外推结果．     

Properties of MgO at high pressures: Shell-model molecular dynamics simulation

Shell-model molecular dynamics (MD) simulation has been performed to investigate the melting of the major Earth-forming mineral: periclase (MgO), at elevated temperatures and high pressures, based on the thermal instability analysis. The interatomic potential is taken to be the sum of pair-wise additive Coulomb, van der Waals attraction, and repulsive interactions. The MD simulation with selected Lewis–Catlow (LC) potential parameters is found to be very successful in describing the melting behavior for MgO, by taking account of the overheating of a crystalline solid at ambient pressure. The thermodynamic melting curve is estimated on the basis of the thermal instability MD simulations and compared with the available experimental data and other theoretical results in the pressure ranges 0–150 GPa. Our simulated melting curve of MgO is consistent with results obtained from Lindemann melting equation and two-phase simulated data at constant pressure by Belonoshko and Dubrovinsky, in the pressure below 20 GPa. The extrapolated melting temperatures in the lower mantle are in good agreement with the results obtained from Wang's empirical model up to 100 GPa. Compared with experimental measurements, our results are substantially higher than that determined by Zerr and Boehler, and the discrepancy between DAC and MD melting temperatures may be well explained with different melting mechanisms. Meanwhile, the radial distribution functions (RDFs) of Mg–Mg, O–Mg, and O–O ion pairs near the melting temperature have been investigated.     

MgSiO3钙钛矿高温高压特性的第一性原理研究

本文采用第一性原理方法，计算了MgSiO3钙钛矿在零温和0 ~ 150 GPa静水压范围内的晶体结构和弹性模量，并利用准简谐近似Debye模型，拟合三阶Birch-Murnaghan物态方程得到了其高温高压下的热力学性质。通过与现有的理论和实验的结果数据比较，确认在0 ~ 2000 K的温度区间内，第一性原理计算结合Debye模型能够较可靠地模拟在下地幔压力范围内MgSiO3钙钛矿的热力学性质。     

Melting and Grüneisen parameters of NaCl at high pressure

The Buckingham potential has been employed to simulate the melting and thermodynamic parameters of sodium chloride (NaCl) using the molecular dynamics (MD) method. The constant-volume heat capacity and Grüneisen parameters have been obtained in a wide range of temperatures. The calculated thermodynamic parameters are found to be in good agreement with the available experimental data. The NaCl melting simulations appear to validate the interpretation of superheating of the solid in the one-phase MD simulations. The melting curve of NaCl is compared with the experiments and other calculations at pressure 0－30GPa range.     

Development of modified embedded atom potentials for the Cu–Ag system

The modified embedded atom method is tested in the atomistic simulations of binary fcc metallic alloys. As an example the alloying behaviour of Cu–Ag is studied using the molecular dynamics (MD) method. The MD algorithms that we use are based on the extended Hamiltonian formalism and the ordinary experimental conditions are simulated using the constant-pressure, constant temperature (NPT) (MD) method. The enthalpy of mixing values of the random Ag–Cu binary alloys are obtained as functions of concentration after 20 000 steps.     

Energy control of a molecular dynamics cell for the promotion of crystal phase transitions

A method for promoting crystal phase transitions in molecular dynamics (MD) simulations is proposed in which the temperature of a variable MD cell is controlled within the framework of the ParrinelleRahman constant-pressure method. This control is independent of the terqperature control of the molecules. The implementation makes it possible to perform MD simulations in which the temperature of the cell is high enough to enhance the deformation of the cell, while that of molecules is kept low so as not to lose the order of the crystal. The method successfully induced the phase transition of benzene crystals, which has not been obtained in MD simulations under usual conditions. Compared with the previously proposed method, i.e., momentum control of the cell, the present method is more favourable for reproducing experimentally observed structures.     

Molecular dynamics simulations of phase transition of n-nonadecane under high pressure

The phase transition behavior of n-nonadecane under high pressure was investigated with molecular dynamics (MD) simulations method. A simplified model with amorphous structure and periodic boundary conditions in constant-temperature, constant-pressure ensemble was used in this study. The results showed that the whirling and molecules motion of n-nonadecane chains were restrained by the high pressure. The simulated phase transition temperature of n-nonadecane under high pressure is higher than that under atmospheric pressure. The order parameter of n-nonadecane decreases with the increase in temperature. The simulations reveal that MD is an effective method to understand the phase transition of alkane-based phase change materials on molecular and atomic scale.     

Molecular dynamics study of nanoparticle evolution in a background gas under laser ablation conditions

Long-time evolution of nanoparticles produced by short laser interactions is investigated for different materials. To better understand the mechanisms of the nanoparticle formation at a microscopic level, we use molecular dynamics (MD) simulations to analyse the evolution of a cluster in the presence of a background gas with different parameters (density and temperature). In particular, we compare the simulation results obtained for materials with different interaction potentials (Morse, Lennard-Jones, and Embedded Atom Model). Attention is focused on the evaporation and condensation processes of a cluster with different size and initial temperature. As a result of the MD calculations, we determinate the influence of both cluster properties and background gas parameters on the nanoparticle evolution. The role of the interaction potential is discussed based on the results of the simulations.